1. Field of the Invention
The present invention relates to an optical transmission device and an optical phase modulator for enabling increased transmission rate and transmission distance.
2. Description of the Related Art
Recently, large capacity transmission networks using light have been widely used. In an optical transmission system which constitutes such an optical network, an IM/DD (intensity modulation/direct detection) scheme is extensively implemented. In such an IM/DD scheme, intensity of an optical output is modulated, using logic levels of a signal to be transmitted such as “0” and “1” which correspond to states of the optical output such as a “luminescent state” and a “non-luminescent state”, respectively. Then, the received modulated optical signal is photoelectrically converted and directly detected using a photodiode (PD) or the like. As an optical transmission line for use in the optical network, a single-mode optical fiber is commonly employed. However, the wavelength dispersion characteristics of single-mode optical fibers result in an increased pulse width, degradation of the received optical signal, and interference between symbols, thereby imposing limitations on transmission rate and distance.
Techniques related to optical transmission will be described in detail with reference to the attached drawings. FIG. 11 illustrates a direct intensity modulation technique, in which the intensity of an optical output of an LD (laser diode) is directly modulated using an LD drive circuit.
Referring now to FIG. 11, an LD drive current is composed of a bias current (Ib) controlled by an Ib control signal and a modulation current (Ip) controlled by an Ip control signal. The LD drive current is applied to an LD 101. The LD, 101 then produces output light (optical signal) whose intensity is modulated in accordance with a transmission signal.
FIGS. 12A through 12C illustrate output light whose intensity is modulated in accordance with a transmission signal.
FIG. 12A illustrates the transmission signal. FIG. 12B illustrates the intensity-modulated output light. The LD 101 is biased by the Ib and outputs a continuance wave (CW) optical signal which is modulated by the Ip in accordance with the transmission signal. A logic level “0” and a logic level “1” of the transmission signal represent a non-luminescent state and a luminescent state of the output light, respectively. The output light of FIG. 12B can generally be represented by a waveform as shown in FIG. 12C in which the CW is not shown.
In the following, a state of an optical signal transmitting a transmission signal which is at logic level “0” is referred to as a non-luminescent state or “Off”, and a state of an optical signal transmitting the transmission signal which is at logic level “1” is referred to as a luminescent state or “On”.
FIG. 13 illustrates degradation of an optical signal. For example, when a transmission signal over three time slots has a logic level sequence of “101”, an optical receiver, when used in short distance transmission, can distinguish the logic level transitions, as shown in FIG. 13A. However, when the optical signal is received by the optical receiver after long distance transmission, the amplitude of the optical signal becomes smaller at logic level “1”, and the pulse width of the optical signal becomes longer at logic level “1”, as shown in FIG. 13B. This causes the optical receiver to misrecognize the signal element of the logic level “0” in the second time slot as the signal element of the logic level “1”, due to the expanded pulse width of each adjacent signal element of the logic level “1”. The likelihood that this misrecognition occurs increases as a signal interval of one time slot becomes shorter, which results from an increased transmission rate, and as a receiving level for an optical signal becomes shorter, which results from an increased transmission distance.
With a view to overcoming the foregoing, or to realizing a higher transmission rate as well as a longer transmission distance, an optical duo binary technique is proposed by Yonenaga et al. [3], “Optical duo binary transmission system with no receive sensitivity degradation”, Electronics Letters, Vol. 31 No. 4, 16 Feb. 1995, and in Japanese Unexamined Patent Application Publication No. 08-139681. In the proposed duo binary technique, a binary signal having logic levels “0” and “1” is transmitted, so that the signal is recognized to have three logic levels; a logic level “0” and two different logic levels “1”, for each of which the phase of the signal is shifted by π radians.
FIG. 14 is a first diagram illustrating the duo binary technique. FIG. 15 is a second diagram illustrating the duo binary technique. FIG. 16 is a third diagram illustrating the duo binary technique.
In FIG. 14, an LD 101 is configured to be controlled by an LD drive circuit 102 in which a bias current (Ib) is set and to output a CW as a source of an optical carrier. A duo binary encoding circuit 104 generates a duo binary signal 1 and a duo binary signal 2 for producing duo binary codes. The intensity and phase of the CW are modulated by a Mach-Zehnder interferometer-type optical intensity modulator (MZ) 103, in accordance with the duo binary signal 1 and the duo binary signal 2 supplied through the respective drivers 105.
In FIG. 15, for example, in signal timing 0 through signal timing 9, each signal timing corresponding to a time slot, a logic level sequence of a transmission signal is “0101100110”. The logic level sequence of the intensity of an optical duo binary signal that represents output light is “0101100110.” The phase of the optical duo binary signal is indefinite and represented as “−”, when the signal intensity logic level is “0”. When the signal intensity is at logic level “1” in signal timing 1, the phase of the optical duo binary signal is “0”. In signal timings 3, 4, 7, and 8, the signal intensity in each signal timing is at logic level “1”, and the phase of the optical duo binary signal is “1”. This indicates the phase of duo binary signal is configured to be inverted with respect to the phase in signal timing 1, in this case, with signal intensity logic level “1”. In this example, a signal for applying an electric field to a first waveguide of the MZ 103 of FIG. 3 refers to the duo binary signal 1, and a signal for applying an electric field to a second waveguide of the MZ 103 of FIG. 3 refers to the duo binary signal 2. The phase inversion (phase shift by π radians) with respect to the phase of a signal element of logic level “1” is represented as “−1”.
The equation of FIG. 16A illustrates a case where the transmission signal is at logic level “0” (intensity modulation “Off”). A first CW (optical carrier signal) traveling along the first waveguide of the MZ 103 can be represented as COS(ωt+π/2), and a second CW traveling along the second waveguide of the MZ 103 can be represented as COS(ωt−π/2). This indicates that the first CW and the second CW from the first and second waveguides of the MZ 103 are combined, which results in output light whose amplitude is “0”. The equation of FIG. 16B illustrates a case where the transmission signal is at logic level “1” (intensity modulation “On”) with respect to signal timing 1. The first CW traveling along the first waveguide of the MZ 103 is phase modulated so as to be represented as COS(ωt+π/2−π/2), and the second CW traveling along the second waveguide of the MZ 103 is phase modulated so as to be represented as COS(ωt+π/2+π/2). Then, the first CW and the second CW are combined, so as to be represented as 2COS(ωt).
FIG. 16C illustrates a case where the transmission signal is at logic level “1” (intensity modulation “On”) with respect to signal timings 3, 4, 7, and 8. The first CW traveling along the first waveguide of the MZ 103 is phase modulated so as to be represented as COS(ωt+π/2+π/2), and the second CW traveling along the second waveguide of MZ 103 is phase modulated so as to be represented as COS(ωt−π/2−π/2). Then, the first CW and the second CW are combined, thereby being represented as 2COS(ωt+π).
Thus, an optical transmission device for transmitting a ternary optical signal can be realized in such a manner as described above. The ternary optical signal includes three values each of which can be represented by a phase value “0” resulted from a transmission signal at logic level “0”, and two different phase values “2COS(ωt)” and “2COS(ωt+π)” of the CW optical signal, which are obtained where the transmission signal is at logic level “1”.
FIG. 17 illustrates a characteristic of the duo binary technique, with respect to the optical signal generated through the duo binary technique described with reference to FIG. 14 and FIG. 15. For example, where a logic level sequence of a transmission signal over three time slots is “101” similarly to the case of FIG. 13, long distance transmission results in the smaller amplitude and the longer pulse width, as illustrated in FIG. 17B. On the other hand, as shown in FIG. 17A, the phases of the signal elements of logic level “1” in the first and third time slots are inverted with respect to each other. This can compensate the interference caused by the adjacent signal elements of logic level “1,” which may occur in the timing when the transmission signal is at the logic level “0”, in the second time slot, in this case. Thus, the receiver can distinguish the signal element at the logic level “0” without error.
However, the above duo binary technique needs a signal generation procedure for converting a binary signal into a ternary signal and for obtaining a ternary electric signal for driving an optical modulator, resulting in further needs for complicated modulating means and means for driving the modulating means.
Japanese Unexamined Patent Application Publication No. 10-112688 discloses a duo binary technique in view of the above disadvantage. In the duo binary technique, an optical intensity modulator provided in a preceding stage performs intensity modulation on a optical signal, and an optical phase modulator provided in a subsequent stage performs phase inversion on the intensity modulated optical signal which is at logic level “0”.
FIG. 18 illustrates an optical transmission device which implements the duo binary technique. The optical transmission device includes an LD drive circuit 110, an LD 111, an optical intensity modulator 112, an optical phase modulator 113, plural drivers 115, an encoding circuit 121, and a delay circuit 122. The LD drive circuit 110 applies an LD current, in which a bias current (Ib) is set, to the LD 111. The LD 111 oscillates in response to the bias current set to be larger than or equal to a stimulated emission threshold LD current and then generates a continuous wave (CW) optical signal. The optical intensity modulator 112 modulates the intensity of the CW optical signal generated by the LD 111 in accordance with a transmission signal received through the driver 115. More specifically, the intensity of the CW optical signal is modulated so as to be in a non-luminescent state when the transmission signal is at logic level “0”, and to be in a luminescent state when the transmission signal is at logic level “1”. The optical phase modulator 113 modulates the phase of the intensity-modulated CW optical signal in accordance with a phase modulation signal. The encoding circuit 121 generates a control signal (phase modulation signal) for causing the optical phase modulator 113 to invert the phase of the CW optical signal (phase shift by π radians), in response to the logic level “0” of the transmission signal. The delay circuit 122 delays the phase modulation signal, thereby matching the timing of the phase modulation signal with the timing of phase modulator 113.
FIG. 19 illustrates an example of code conversion according to the duo binary technique. In signal timing 0 to signal timing 9, a logic level sequence of a transmission signal over ten time slots is “0101100110”. Under this condition, the logic level sequence of the intensity of output light is “0101100110”, in which “0” and “1” indicate a non-luminescent state and a luminescent state, respectively. The phase of the output light represents code conversion in which the phase of a CW optical carrier signal is inverted (shifted by π radians) with respect to a preceding phase, when the transmission signal is at logic level “0”. For example, in signal timing 0, the logic level of the output light intensity is “0”, and the phase is inverted to “0”. In signal timing 1, the logic level of the output light intensity is “1”, and thus the phase is held to be “0”. In signal timing 2, the logic level of the output light intensity is “0”, and thus the phase is inverted (shifted by π radians). In signal timing 3 and signal timing 4, the logic level of the output light intensity is “1”, and thus the phase is held to be “π”. In signal timing 5, the logic level of the output light intensity is “0”, and thus the phase is inverted to “0”. Likewise, in signal timing 6 and signal timing 9, the phase is inverted, and in signal timing 7 and signal timing 8, the phase in the previous signal timing is maintained. The encoding circuit 121 generates a phase modulation signal whose logic level is shifted when the transmission signal is at logic level “0”, thereby controlling the optical phase modulator 113.
FIG. 20 illustrates signal degradation that occurs in the duo binary technique of FIG. 19. In FIG. 19, in signal timing 1 and signal timing 3, the phases of the optical signal elements at logic level “1” of the output light are inverted with respect to each other. On the other hand, in signal timing 4 and signal timing 7, the phases of the optical signal elements at logic level “1” of the output light are identical, or π radians. Thus, as illustrated in FIG. 17C, the phases of the CW optical signal elements in signal timing 1 and signal timing 3 are inverted with respect to each other. This cancels the interference that occurs in signal timing 2 in this example, or when the signal is at logic level “0”, due to the adjacent signal elements at logic level “1”, which enables an optical receiver to distinguish each logic level without error. However, in signal timing 4 and signal timing 7, the phases of the CW optical signal elements in each timing are identical, or π radians, leaving the cancellation of the interference insufficient.
As described above, the optical transmission device employs the duo binary technique, in which an optical intensity modulator arranged at a preceding stage modulates the intensity of an optical signal and in which an optical phase modulator arranged at a subsequent stage inverts the phase of the intensity modulated optical signal in response to an optical signal intensity logic level “0” (in a non-luminescent state). In such an optical transmission device, when a logic level sequence of a transmission signal over four time slots is “1001”, an optical signal element which corresponds to a transmission signal component of logic level “0” (non-luminescent state) may be misrecognized as being at logic level “1” (luminescent state) due to an extended pulse width. This phenomenon has imposed a limitation on increases in transmission rate and distance. More specifically, as a transmission rate increases, a time slot interval becomes shorter. As a transmission distance increases, a receiving level for an optical signal becomes lower, making transmission vulnerable to interference between symbols.